ULTRAFILTRATION OF A POLYMER-ELECTROLYTE MIXTURE

We present a mathematical model to describe the ultrafiltration behavi our of polymer-electrolyte mixtures. The model combines the proper the rmodynamic forces (pressure, chemical potential and electrical potenti al differences) with multicomponent diffusion theory. The model is ver ified with experimental data on the ultrafiltration of aqueous solutio ns of PEG-4000 and potassium phosphate. The single solute rejection of PEG-4000 goes through a maximum as also found by others. The single s olute rejection of potassium phosphate depends on the ionic strength o f the solution. At low ionic strength rejections are found of 50%. Sol utions containing a high concentration of PEG-4000 and potassium phosp hate show a negative rejection for potassium phosphate. This is caused by the strongly non-ideal behaviour of these aqueous solutions. The m odel predicts the behaviour of single solute experiments quite well, b ut some deviations are found with the mixed solute experiments. Howeve r, negative rejections found in the mixed solute experiments are predi cted by the model.